1. Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO) wireless communication system. More particularly, the present invention relates to an apparatus and method for detecting a signal by considering an interference signal from a neighboring cell in a MIMO wireless communication system.
2. Description of the Related Art
Recently, significant attention is paid to the Multiple Input Multiple Output (MIMO) technique, and active research on the MIMO technique is currently being conducted. The MIMO technique performs communication by using a plurality of streams through a plurality of antennas. Therefore, with this technique, a channel capacity can be remarkably improved in comparison with a case of using a single antenna. For example, if both a transmitting end and a receiving end use M Transmit (Tx)/Receive (Rx) antennas in a case where channels between the antennas are independent, and a bandwidth and an overall transmission power are fixed, then an increase in an average channel capacity is M times higher in comparison with the case of using a single antenna.
Typical examples of a signal receiving method used in a MIMO wireless communication system include a Minimum Mean Square Error (MMSE) method and a Maximum Likelihood (ML) method. When using the MMSE method and the ML method, a Tx signal is determined by considering Additive White Gaussian Noise (AWGN). Specifically, in the MMSE method, the Tx signal is determined by multiplying an Rx signal by a specific nulling matrix. In this case, the nulling matrix is selected so that a Mean Square Error (MSE) value between the detected Tx signal and the Rx signal is minimized. In the ML method, all possible combinations of Tx signals are compared with an Rx signal, so as to select the most similar Tx signal.
In the MMSE method, the nulling matrix is computed by using Equation (1) below.W=(HH+σ2InT)−1HH  (1)
In Equation (1), W denotes a nulling matrix, H denotes a channel matrix, σ denotes a standard deviation of noise, InT denotes a unit matrix having a size of nT, and nT denotes the number of Tx antennas.
In the ML method, if k Tx signals are used, an approximate value between possible combinations of Tx signals and the RX signal is computed by squaring distances as expressed by Equation (2) below.∥y−H{circumflex over (x)}∥=∥h0(x0−x0[i])+ . . . +hk-1(xk-1−xk-1[j])+n∥2  (2)
In Equation (2), y denotes a received signal vector, H denotes a channel matrix, {circumflex over (x)} denotes a Tx signal detected by a receiving end, hk denotes a channel vector for a kth Tx signal, xk denotes the kth Tx signal, xk[m] denotes an mth transmittable signal for the kth Tx signal, that is, the number of constellations, and n denotes a noise vector. That is, the receiving end conforming to the ML method determines a Tx signal among possible combinations of Tx signals when a result of Equation (2) above is minimized.
As described above, a signal transmitted through a plurality of antennas can be detected by using the MMSE or ML method. However, in a conventional cellular system, a neighboring cell causes not only noise but also interference. That is, as shown in Equation (1) and Equation (2) above, when only noise is taken into account in the MMSE and ML methods, there is a high possibility that the interference results in deterioration of system throughput. Therefore, there is a need for a method of detecting a signal by considering not only noise but also interference from a neighboring cell.